Shallow lakes play an important role in the global carbon cycle, and can bury substantial quantities of organic carbon in their sediments. However, sedimentation rates vary greatly among shallow lakes and the factors controlling these rates are poorly known. Variability in sedimentation rates could be partially due to differences in decomposition rates, with decomposition driven by differences in lake depth, dissolved oxygen levels, temperature, and dominant primary producer. We tested the influence of water depth, type of primary producer, and whole lake effects on decomposition rates by deploying litter bags with four types of primary producers at two depths (0.75m and 1.5m) from October through August 2010 in 11 shallow lakes in western Minnesota, USA. The four types of primary producers were the filamentous algae Cladophora sp., the submerged angiosperm Stuckenia pectinatus (sago), the submerged macroalgae Chara sp., and the terrestrial grass Panicum virgatum.
Results/Conclusions
Results showed significant differences in decomposition rates among species, depths, and at the whole-lake scale. Surprisingly, sago showed the slowest decomposition rate, grass and Cladophora were moderate, while Chara decomposed the fastest, with percent dry-mass remaining after 293 days of 25%, 10%, 6% and 2%, respectively. Remaining mass averaged across species was twice as high at the shallow depth (10%) compared to the deep depth (5%) despite a depth difference of just 0.5m. Differences at the whole-lake scale were large, as lakes ranged from 19% to 3% remaining mass averaged across depths and species. However, we failed to find any factors associated with differences at the whole-lake scale. Our results indicate yearly decomposition rates are influenced by water depth and type of primary producer, and also that rates differ at the whole-lake scale regardless of depth or type of organic material. Factors responsible for the whole-lake differences remain elusive.